The effect of endogenous thyroxin-binding proteins on the measurement of free thyroxin (FT4) by five different methodologies represented in 16 different assays was studied in patients with nonthyroidal illness (NTI). All FT4 assays correlated with the serum concentration of some or all of the three major T4-binding proteins (prealbumin, albumin, and TBG). The FT4 estimates varied with methodology and to a lesser extent with the particular assay (kit) used. The results from two-step FT4 RIAs were least associated with binding protein concentrations.